The infusion of fluids such as parenteral fluids into the human body is accomplished in many cases by means of a syringe pump having a lead screw on which a screw drive mechanism is mounted. Rotation of the lead screw is translated into linear motion by the screw drive mechanism. The linear motion is transmitted to a syringe plunger by a plunger driver that is typically rigidly connected to the screw drive mechanism. To facilitate replacement of an empty syringe, split nuts or half nuts are used as part of the screw drive mechanism in some syringe pumps for easy and rapid repositioning of the screw drive mechanism and plunger driver on the lead screw to engage the syringe plunger of a full syringe with the plunger driver. An activating lever or other mechanical interface for the operator is provided at the plunger driver to separate the split nut threads from the lead screw threads and to separate the plunger driver from the present syringe plunger. The plunger driver is then manually moved away from the empty syringe so that it can be removed. In the case where a new syringe is to be loaded into the syringe pump, the operator continues to manipulate the activating lever or other mechanical interface to disengage the split nuts from the lead screw, move the plunger driver and split nuts along the lead screw to an appropriate position, and inserts the new syringe into the pump. The operator then moves the plunger driver into contact with the plunger flange of the new syringe, releases the activating lever when the plunger driver is pressed against the syringe plunger, and thereby allows the split nuts to drop onto the lead screw. Ideally, the crests of the split nut threads drop as far as possibly into the troughs of the lead screw threads so that the threads of both devices are fully engaged. Unfortunately, the threads do not always fully engage.
As shown in FIG. 1, in some cases the crests of the split nut threads 10 undesirably align with and contact the crests of the lead screw threads 12 rather than the threads engaging each other. Because either set of the threads may be relatively flat at their crests, they do not engage each other until the lead screw has turned a sufficient amount and a bias device of the screw drive mechanism has forced the split nut threads to drop into and engage the lead screw threads. Under low infusion rates, such sufficient lead screw rotation may take a substantial amount of time during which the patient receives no infusion fluid.
In another case as presented in FIG. 2, the non-driving surfaces of the split nut threads 10 may undesirably contact the non-driving surfaces of the lead screw threads 12 when the activating lever is released. The arrow indicates the direction of forward movement of the split nuts and plunger driver. Due to the thread clearance 14 between the two sets of threads, some amount of time may pass before the driving surfaces of the lead screw threads 12 fully engage the driving surfaces of the split nut threads 10 and actually begin to move the syringe plunger along with the split nuts. This amount of time also would be a period during which the patient receives no infusion fluid.
In yet another case as presented in FIG. 3, the driving surfaces of the split nut threads 10 may only partially engage the driving surfaces of the lead screw threads 12. Partial engagement occurs when the crests of the split nut threads 10 fail to drop completely into the troughs of the lead screw threads 12. When this occurs, a large amount of back pressure exerted by the full syringe and a force vector developed by the lead screw threads 12 tend to force the split nut threads 10 up and out of the lead screw threads 12. If the biasing force on the split nuts is insufficient to overcome these factors, the split nut threads 10 may be forced out of contact with the driving surfaces of the lead screw threads 12 and into the position shown in FIG. 1. This case would likewise result in a time period during which the patient would receive no infusion fluid and this time period may exceed that associated with FIG. 1.
Under low infusion rates, it could be an hour or more before non-engagement or partial engagement conditions illustrated in FIGS. 1-3 and the resulting failure to deliver infusion fluid are discovered and corrected or automatically overcome by the slow rotation of the lead screw. This problem is only partially overcome in a prior system that automatically rotates the lead screw in a forward direction to remove any thread clearance or in a combination of reverse and forward directions to fully seat the split nut threads then remove any thread clearance. An engagement sensor is employed to give a positive engagement signal when the split nut threads were seated within in the lead screw threads at or below a point corresponding to a percentage of root thread depth deemed to represent sufficient engagement. If the engagement signal was positive, the lead screw is automatically rotated a fixed amount in the forward direction to remove any thread clearance. This occurs even when the split nut threads 10 happen to be already fully seated. Thus, the automatic rotation could move the syringe plunger along with split nuts and cause excessive fluid infusion. If the engagement signal was negative, the lead screw is rotated a fixed amount in the reverse direction and another reading of the engagement sensor is taken. If the engagement signal remained negative, the lead screw would be rotated again by some fixed amount in the reverse direction and a reading of the engagement sensor is taken again. The reverse rotations would be repeated a fixed number of times, after which the lead screw would be rotated by some fixed amount in the forward direction followed by another reading of the engagement sensor. An alarm would be set off after a fixed number of unsuccessful attempts at thread engagement. If after any attempt the split nut threads were deemed sufficiently seated, the lead screw would be automatically rotated a fixed amount in the forward direction to remove any possible thread clearance. Thus, the prior system always rotates the lead screw by a fixed amount independent of the linear position of the crests of the split nut threads relative to the crests of the lead screw threads.
In addition, the automatic rotation in the prior system is initiated after a signal from a plunger sensor indicates that a syringe plunger is present and after a signal from the engagement sensor indicates that the split nut threads 10 are not sufficiently seated. Undesirably, the rotation occurs prematurely in some instances when a negative indication from the engagement sensor arises from the operator having failed to release the activating lever, not because the crests of the split nut threads 10 are resting on the crests of the lead screw threads 12.
Thus, when the automatic rotation routine has completed and the operator subsequently releases the activating lever, the two sets of threads may again be in one of the positions shown in FIGS. 1-3 and result in delayed fluid infusion. The resulting delayed and excessive fluid infusion associated with syringe replacement as may occur in prior systems is undesirable under low infusion rates where more precise and accurate control of fluid infusion is required.
Hence, those skilled in the art have recognized a need for an improved lead screw engagement system that reduces the possibility of delayed and excessive fluid infusion associated with syringe mounting in the pump. The present invention fulfills this need and others.